1. Field of the Invention
The present invention relates to a four-wheel-drive-vehicle driving force distribution apparatus for switching an auxiliary transmission mechanism and changing the pressing force of a friction clutch, which transmits motive power to front wheels, in a non-step manner and, particularly, relates to a four-wheel-drive-vehicle driving force distribution apparatus for switching the auxiliary transmission mechanism and changing the pressing force of the friction clutch by one motor.
2. Description of the Related Arts
Conventionally, in a driving force distribution apparatus for a four-wheel-drive vehicle having an auxiliary transmission mechanism and a friction clutch which controls torque distribution to front wheels in a non-step manner, switching of the auxiliary transmission mechanism and pressing force control of the friction clutch has been performed by one motor (actuator).
For example, in the drive force distribution apparatus of Japanese Kohyo Patent Publication No. 2004-514587 for a four-wheel-drive vehicle, a mechanism which switches output directions of a motor by an electromagnetic clutch when switching of an auxiliary transmission mechanism is to be performed or when control of a friction clutch is to be performed is provided so as to perform the switching and control by one motor. In the driving force distribution apparatus of U.S. Pat. No. 6,645,109 for a four-wheel-drive vehicle, a shift fork for switching an auxiliary transmission mechanism and a pinion gear for driving a ball cam are fixedly disposed coaxially on a drive shaft which is driven by a motor, and pressing force of a friction clutch is controlled by drive of the ball cam in the rotation area in which a shift groove of the cylindrical cam is not displaced by rotation. Furthermore, in the driving force distribution apparatus of Japanese Patent Application Laid-Open (kokai) No. 2004-249974 for a four-wheel-drive vehicle, a shift fork for switching auxiliary transmission mechanism and a pinion gear for driving a ball cam are fixedly disposed coaxially on a drive shaft, which is driven by a motor, so that the shift fork and the pinion gear are driven with a certain angle of looseness by cam followers provided on the motor drive shaft. However, such conventional driving force distribution apparatuses for four-wheel-drive vehicles have the following problems. The driving force distribution apparatus of Japanese Kohyo Patent Publication No. 2004-514587 for a four-wheel-drive vehicle has a problem that the cost thereof is high since the electromagnetic clutch is needed, although the apparatus can be made with one motor. The driving force distribution apparatus of U.S. Pat. No. 6,645,109 for a four-wheel-drive vehicle can perform switching of the auxiliary transmission mechanism and control of the friction clutch by one motor without using an electromagnetic clutch. Generally, in switching of the auxiliary transmission mechanism, the situation that the meshing phases of clutch gears are misaligned and the shift fork cannot be moved to a predetermined position may occur. However, in U.S. Pat. No. 6,645,109, since the shift fork is directly driven by the cylindrical cam provided on the drive shaft, there is a crucial problem that shifting cannot be performed when the clutch gear meshing phases of the auxiliary transmission mechanism are misaligned. Moreover, since the cylindrical cam for shifting and the pinion gear for driving the friction clutch are integrally rotated, the gear for driving the friction clutch is rotated even during a shifting period. Therefore, there is a problem that the size of the drive gear for the ball cam mechanism and cost are increased. The drive force distribution apparatus of Japanese Patent Application Laid-Open (kokai) 2004-249974 for a four-wheel-drive vehicle has a crucial problem that shifting cannot be performed when the clutch gear meshing phases of the auxiliary transmission mechanism are misaligned since the shift fork is directly driven by the cam followers which are provided on the drive shaft. Regarding shift control and friction clutch control, when the drive angle that is used for obtaining the same control force is increased, the drive torque of the motor can be reduced. However, in Japanese Patent Application Laid-Open (kokai) 2004-249974, the structure thereof utilizes cam surfaces provided on shift fork end faces. Therefore, the torque of the motor has to be increased since there is a restriction that the rotation angle of the drive shaft cannot be increased more than 360 degrees, and there is a problem that the size of the motor is increased which leads to increased cost. Furthermore, as a problem common to U.S. Pat. No. 6,645,109 and Japanese Patent Application Laid-Open (kokai) 2004-249974, as shown in FIG. 1, in order to perform pressing force control of the friction clutch with respect to both a H position and a L position of the shift of the auxiliary transmission mechanism, a drive gear of the ball cam mechanism for changing the pressing force of the friction clutch has to be driven in both directions, forward rotation and reverse rotation.
FIG. 2A shows a drive gear 202 of a ball cam mechanism 200 which changes clutch pressing force merely by rotation toward the H-position side of an auxiliary transmission mechanism. On the other hand, FIG. 2B shows a drive gear 204 of a ball cam mechanism 200 which changes clutch pressing force for both the H position and the L position, wherein there is a problem that the size of the gear part and the weight thereof is increased, and cost is also increased. In addition, there is also a problem that the gear part cannot be accommodated in a case.